Exsolution Willemite from Sterling Hill Mine, New Jersey
Contributed by: Michael Crawford
Date: Jun 21st, 2025
Locality: Sterling Hill, Ogdensburg, Sussex County, New Jersey, USA (See on Mindat)
Size: 10 x 11 cm
Description:
This example demonstrates how fluorescence can reveal exsolution structures. Exsolution is a process that initiates with a homogeneous solid solution at elevated temperatures. As the mineral solution cools, it separates into two or more distinct phases. The solid solution contains mineral components of similar chemical composition.
The specimen under consideration contains willemite (Zn2SiO4) and tephroite (Mn2SiO4). During the cooling of the solid solution of willemite and tephroite, the mineral phases separated due to the atomic size disparity between zinc and manganese. Stresses in the cooling mass likely facilitated the formation of parallel lamellae zones of willemite. This exsolution texture is challenging to observe in white light images. However, under shortwave UV illumination, the willemite lamellae exhibit bright green fluorescence, whereas tephroite does not fluoresce. The fluorescence of the lamellae is faint under longwave and midwave UV illumination. The lamellae are less than 0.5 mm thick. Additionally, the specimen contains some red fluorescent calcite. Willemite shows maximum shortwave fluorescence at 521 nm, while calcite’s maximum shortwave fluorescence occurs at 620 nm.
This specimen originates from the Sterling Hill Mine, Ogdensburg, Sussex County, New Jersey.
Summary of luminescence responses:
Willemite
- Fluorescence under Shortwave (255nm LED) UV light: Green
- Fluorescence under Longwave (365nm LED) UV light: Red
- Fluorescence under Midwave (305nm LED) UV light: Red
- Fluorescence under Shortwave (255nm LED) UV light: Red